Custom labelling of equations in appendix section

Question

I spent some time looking at existing posts on this topic but I could not find anything that I was able to use. I am writing a LaTeX document with some sections and an appendix. I would like all the equations in the sections to be labeled (and cross-referenced) normally, using the \label{somelabel} inside \begin{equation} ... \end{equation} and then \ref{somelabel} when I want to reference it. So this will produce labels (1), (2), etc.

But I would like to use a separate counter for the equations in the appendix, and these equations should be labeled (A1), (A2), etc. So the printed output should look like:

Section 1 This is the first equation (1). Using equation (A1), we find the second equation (2)

Section 2 This is the third equation (3), using equation (A2)

Appendix This is the first appendix equation (A1). This is the second appendix equation (A2).

etc.

Answer 1

I suggest you add the instructions

\counterwithin*{equation}{section}
\renewcommand\theequation{\thesection\arabic{equation}}

immediately after \appendix. The former instruction tells LaTeX to reset the equation counter to zero each time \section is executed. The latter determines how equation numbers will be displayed -- here: by prefixing the section "number" (A, B, etc) before the plain equation number.

That way, equations in Appendix B will automatically be labeled "B1", "B2", etc.

\documentclass{article}
\usepackage[noabbrev]{cleveref} % for "clever" cross-references

\begin{document}

\noindent 
\Cref{eq:aaa,eq:bbb}.
\Cref{eq:ccc,eq:ddd,eq:eee,eq:fff,eq:ggg,eq:hhh}.

\section{Hello World}

\begin{equation} \label{eq:aaa} 1+1=2 \end{equation}
\begin{equation} \label{eq:bbb} 1+2=3 \end{equation}

\appendix  % label section numbers alphabetically: "A", "B", etc
\counterwithin*{equation}{section} % reset 'equation' counter whenever '\section' is executed
\renewcommand\theequation{\thesection\arabic{equation}} % how to display the equation "number"

\section{Uno}
\begin{equation} \label{eq:ccc} 2+2=4 \end{equation}
\begin{equation} \label{eq:ddd} 2+3=5 \end{equation}
\begin{equation} \label{eq:eee} 3+3=6 \end{equation}

\section{Due}
\begin{equation} \label{eq:fff} 3+4=7 \end{equation}
\begin{equation} \label{eq:ggg} 4+4=8 \end{equation}
\begin{equation} \label{eq:hhh} 4+5=9 \end{equation}

\end{document}

Answer 2

A naive, but working version:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\section{One}

\begin{equation}
\label{eq1}
2+2=7
\end{equation}

\begin{equation}
\label{eq2}
2+2=7
\end{equation}

\begin{equation}
\label{eq2}
2+2=7
\end{equation}

\section{Two}

\begin{equation}
\label{eq4}
2+2=7
\end{equation}

\begin{equation}
\label{eq5}
2+2=7
\end{equation}

\begin{equation}
\label{eq6}
2+2=7
\end{equation}

\eqref{eq1}, \eqref{eq4}, \eqref{eq7}

\appendix

\section{Appendix}
\renewcommand{\theequation}{A.\arabic{equation}}
\setcounter{equation}{0}

\begin{equation}
\label{eq7}
2+2=7
\end{equation}

\begin{equation}
\label{eq8}
2+2=7
\end{equation}

\begin{equation}
\label{eq9}
2+2=7
\end{equation}

\eqref{eq1}, \eqref{eq4}, \eqref{eq7}

\end{document}

If you really do not want the dot between A and the number, you can write \renewcommand{\theequation}{A\arabic{equation}}.

Answer 3

The easiest way is to add the section Appendix for equation numbering after you declare the start of \appendix. Like this

\appendix
\numberwithin{equation}{section}

Then all equations after that will be labeled (A.1), (A.2) and so on. This is neat since \numberwithin does not have to be put in the preamble.